I don't understand why people keep bringing this up. Unbrellas in no way make it "unfair" for anyone else, because everyone can use them. If it's the time your worried about it won't speed up a thing.
Do you really think that if eliminate umbrellas a person that normally uses one and steps up to a target in a field on a windy day is just gonna toe the stake and fling an arrow. Don't count on it. They are gonna take as much time as they need so they can shoot when the wind is calm. Some may even go over their allotted time limit. Who is gonna be there to make sure nobody does? Exactly, nobody. So just leave the umbrella rule the way it is and let whoever wants to use one use it.